A terminal in a conventional wireless communication system has a communication mode and a stand-by mode. In the communication mode, the terminal is connected to a base station and communicates with a network. In the stand-by mode, the terminal is not connected to the base station, but monitors neighboring cells regularly and sets one of the cells that can be recognized as a serving cell. When starting a call or receiving a call from the network, the terminal connects to the base station that forms the serving cell, and starts communication. Here, a service area for wireless communication formed by the base station is called cell.
The terminal selects a cell that becomes the serving cell next based on cell selection information included in system information broadcast from the serving cell, the reception power of the serving cell, and the reception power of the neighboring cell(s). The cell selection information includes serving cell information (ServingCellInfo), inter-frequency information (InterFreqCarrierFreqInfo), and neighboring cell information (InterFreqNeighCellInfo). The serving cell information includes parameters such as the “hysteresis for giving priority to the serving cell” (q-Hyst), the “parameter for determining the priority among frequencies” (cellReselectionPriority), and the “threshold of the reception power of the frequency of the serving cell” (threshServingLow).
The inter-frequency information includes parameters such as the “threshold that is of the reception power of a given frequency and referred to when the priority is high” (threshX-High), the “threshold that is of the reception power of the given frequency and referred to when the priority is low” (threshX-Low), the “parameter for determining the priority among frequencies” (cellReselectionPriority), and the “offset of the reception power set for a given frequency” (q-OffsetFreq). The neighboring cell information includes parameters such as the “ID of a neighboring cell” (cell ID) and the “offset of the reception power set for a given neighboring cell” (q-OffsetCell).
The terminal in the stand-by mode sets the next serving cell according to the following (1) to (3).
(1) If the “cellReselectionPriority” in the serving cell information is less than the “cellReselectionPriority” in the inter-frequency information, the terminal in the stand-by mode measures the reception power of a neighboring cell of a different frequency included in the neighboring cell information. If the reception power exceeds the “threshX-High” in the inter-frequency information, the terminal can recognize the neighboring cell that uses a frequency having a high priority. In this case, the terminal sets the neighboring cell as a new serving cell.
(2) If the “cellReselectionPriority” in the serving cell information is equal to the “cellReselectionPriority” in the inter-frequency information, the terminal in the stand-by mode measures the reception power of the current serving cell (S_serv) and the reception power of a neighboring cell of a different frequency (S_neigh) included in the neighboring cell information. If S_neigh satisfies the equation below, it means that S_neigh is higher than S_serv by the offset. In this case, the terminal sets the neighboring cell as a new serving cell.[S_serv]+[q-Hyst]<[S_neigh]−([q-OffsetFreq]+[q-OffsetCell]
(3) If the “cellReselectionPriority” in the serving cell information is greater than the “cellReselectionPriority” in the inter-frequency information, the terminal in the stand-by mode measures S_serv and S_neigh. If S_serv is less than the “threshServingLow,” the terminal determines that the current serving cell is unconnectable. On the other hand, if S_neigh exceeds the “threshX-Low” in inter-frequency information, the terminal determines that the neighboring cell is connectable. In this case, the terminal sets the neighboring cell as a new serving cell.
To summarize (1) to (3) described above, if the terminal in the stand-by mode can recognize multiple cells of different frequencies, the terminal preferentially sets a cell using a frequency of a high priority as the serving cell. If the cells have the same priority, the terminal preferentially sets a cell having a high reception power as the serving cell.
On the other hand, it has been suggested to connect a terminal to a base station as follows: the terminal divides connectable base stations into two groups, namely, a group includes base stations having the high reception power and the other group includes base stations having the low reception power, requests a connection to a base station selected from one of the groups, and if the base station is unconnectable, requests a connection to a base station selected from the other group, and repeats the above operation until the terminal succeeds to connect to a base station. Concerning such conventional communication systems and methods, refer to Japanese Laid-Open Patent Publication No. H11-285046 and 3GPP TS 36.304 V9.1.0 (2009-12).
However, according to the conventional method of setting the serving cell, the terminal in the stand-by mode preferentially sets a cell using a frequency of a high priority as the serving cell, and if the priority is the same, a cell having a high reception power. Generally, the reception power at the terminal in the stand-by mode is greater in a cell that uses a lower frequency band if the transmission power of the base stations is the same. Thus, if multiple terminals in the stand-by mode are located in the same area where multiple cells of different frequencies can be recognized, the terminals are likely to set the same cell as the serving cell. This holds true whether or not the cells are formed by the same base station or by multiple base stations, respectively. This results in an increased load of the cell set as the serving cell in processing connections from the terminals.